Non-Maxwellianity in Temperature Relaxation between Plasma Species

Temperature relaxation between plasma species is usually described by Landau--Spitzer (LS) theory, which assumes Maxwellian distributions throughout. We show analytically that, for two Maxwellian species in homogeneous isotropic plasmas, the interspecies collisions driving relaxation generate a universal three-lobe initial distortion and that these distortions initially reduce energy exchange over a broad parameter range. Full Fokker--Planck simulations reveal substantial relaxation delays when interspecies deformation competes with self-thermalization. For post-shock deuterium--tritium plasmas relevant to inertial confinement fusion, with $T_{\mathrm{T}}/T_{\mathrm{D}}=1.5$, the relaxation time exceeds the LS prediction by 47%. The accompanying distortions lower the apparent DD/DT temperature ratio and produce positive DD and negative DT Gamow-shift residuals relative to Maxwellian predictions. These results also motivate examining non-Maxwellianity in fuel heating by fusion $α$ particles.